Frequently asked questions (FAQs)

Council recognises the importance of accessibility and inclusion in engagement and we are committed to continuously improving in this area to make engaging as easy as possible for all in our community.

Our team are available to:

• assist with understanding the documents
• provide alternative formats if required
• and arrange interpreter support.

Maps of the Study area are available online and in printed formats at our Administration Building Customer counter and at each of our 8 libraries. Customer and Library staff can help you use the maps to identify your property. 

A printable copy of our consultation FAQs is also available, should you prefer in this format.

If you need support to provide feedback during the exhibition, please reach out to the Community Engagement team on:

Email: engagement@ssc.nsw.gov.au
Phone: 02 9710 0333

Whether you'd like a quick chat with the project team or the opportunity to explore the revised Draft Study in more detail, we're offering a range of opportunities to learn more, ask questions and share feedback.

Community pop-up sessions provide an informal opportunity to:

• learn more about the Study.
• understand how to access maps and information.
• ask general questions and share feedback with the project team.

Community information sessions provide a more detailed opportunity to:

• speak directly with Council staff and project specialists.
• explore the revised Draft Study in greater detail.
• discuss property-specific or technical questions.
• review mapping and supporting information.
• share feedback with the project team.

All are welcome. We are committed to making participation accessible and inclusive.

If you would like to discuss alternative ways to provide feedback or require assistance to participate, please contact us on 02 9710 0333 or via engagement@ssc.nsw.gov.au

Your privacy is important to us. It is protected by both Council and our online engagement provider, Granicus, which delivers the Join the Conversation platform.

Council will:

• only use your email address to provide updates or invite you to participate in consultations if you choose to opt in during registration
• not share your personal details with third parties or use them for any other purpose
• manage your information in-line with applicable privacy laws and Council policies.

Granicus is also required to handle your information in accordance with privacy and security requirements.

You can read more about how your information is collected, stored and used by reviewing Council’s Privacy Policy, available on our website.

An Overland Flow Flood Study looks at how rainwater flows across the ground during different types of storms. It shows where water is likely to travel, how deep it may get, and which properties, roads, and other infrastructure could be affected.

To do this, engineers collect information about the area, including the shape of the land, buildings, roads, vegetation, and Council's stormwater drainage system such as pipes, pits and culverts.

This information is then entered into a computer program, which simulates how water moves across the landscape during different rainfall events. The model is tested and refined using information from past floods, including photos, videos, flood marks and observations collected by Council, the State Emergency Service and local residents. Community feedback is valuable because it helps improve our understanding of how flooding behaves in real storms.

The flood model can be used to estimate flood depth, flow speed and the extent of flooding for a range of storm events, from common storms to rare and extreme events.

The Study helps Council understand flood risks across the community. This information can then be used to improve flood planning, emergency management, infrastructure design and future development decisions.

In NSW, councils are responsible for local stormwater drainage and are primarily responsible for managing flood risk to their communities. The NSW Government provides councils with technical and financial support to manage the flood risk through the Floodplain Management Program provided by Department of Climate Change, Energy, the Environment and Water (DCCEEW). The Program exists to understand and reduce the impacts of flooding and flood liability on communities and individual owners and occupiers of flood prone property.

As part of the Floodplain Management Program, councils develop and implement flood risk management plans consistent with the NSW Flood Risk Management Manual (2023). Flood Risk Management Plans outline how councils plan to manage flood risk in the area including any feasible recommended measures, such as upgrading stormwater drainage systems.

Flood studies are the first key step in developing a flood risk management plan. They develop information about flood behaviour to help Council, Government, and the community understand flooding, so they can be informed about flood risks when making decisions and in responding to flood events. 

Flooding in the Sutherland Shire can result from rivers, creeks or the coast, however, the most common flooding which residents experience is overland flow flooding. ‘Overland flow’ typically occurs when rainwater flows over a surface, such as lawns, roads, roofs, car parks and natural areas, on its way to the nearest drain, creek or river. Overland flow flooding can occur during heavy rainfall when stormwater systems temporarily cannot capture all runoff, causing water to flow across natural low points in the land, open channels and drains, roads and footpaths public reserves and even private property.

The first step in managing flood risk is to identify where the risks currently exist. Overland flow can cause flooding, damage to property and create safety risks. Importantly, it can affect areas not typically considered at risk, such as properties on hills or far from creeks or rivers.

These risks are considered in planning for future development and in the design of homes and infrastructure, including improved drainage and higher building levels to reduce flood damage. The information also provides important information to emergency services such as the NSW State Emergency Service (SES).

The NSW Government provides the NSW Flood Risk Management Manual and Guidelines to help councils and experts carry out flood studies, for all kinds of floods including riverine, coastal and overland flow.

Council has engaged BMT, a specialist flood consultant, to prepare this Flood Study. BMT has completed similar studies for other councils across NSW using these guidelines.

The study is overseen by the Flood Risk Management Committee. This group includes community representatives, Sutherland Shire Councillors and technical experts from the NSW Government, the SES and Council.

The first step in managing flood risk is to identify where the risks currently exist. Flood modelling was carried out to understand how water flows during different types of storms. This included checking the model against data from past flood events to improve accuracy. It looked at a range of rainfall events, from smaller, more frequent storms through to rare and extreme events. It also considers potential changes in climate to understand how flood risk may look in the future.

Once modelling was completed the mapping was adjusted so that very shallow depths and/or low velocities of flow are not mapped because these do not pose a risk to people or properties. This process is called “filtering”. Filtering criteria was used to finalise flood mapping to ensure that flood extents are consistently applied based on a risk management approach. Using filtering criteria ensures that properties that experience overland flow are identified so that risk can be managed in development and property protection.

An adopted Overland Flow – Flood Study can be used to prepare Flood Risk Management Studies and Plans, which would allow Council to better manage flood risk for our community. Studies prepared under the NSW Floodplain Management Program may also be eligible for government funding to assist in carrying out important flood mitigation works.

Project milestones and key dates

Yes, the first Draft Overland Flow - Flood Study was exhibited in October 2023 to March 2024. 

The insights and lived experiences received from our local community during the first consultation phase were central to considerations that have shaped our 2026 Draft Study, helping to plan for a more resilient future for everyone.

Click here to see the previous draft Study exhibition webpage

We are now looking forward to speaking with our community about our current 2026 Draft Study, answer questions and capture all feedback as we continue to work with our local community and stakeholders to finalise the Study.

Since the first stage of consultation on the Draft Study, the project team have taken the following actions:

• Reviewed and considered all feedback received, including feedback about the Draft Study and opportunities to improve the way we engage and communicate.
• Conducted field inspections to verify model inputs such as piped infrastructure, ground levels and building footprints. The verified information was used in the updated flood model.
• Engaged an independent peer reviewer to confirm that modelling parameters are consistent with current best practice.
• Updated the rainfall data to align with current technical advice from the Australian Government under Australian Rainfall and Runoff Guidelines.
• Reduced the freeboard applied to the flood planning area from 500mm to 300mm
• Undertook a detailed review of model results to identify irregularities, followed by targeted site inspections to verify (sometimes called “ground-truth” in the Study) and validate the data.
• Reviewed properties identified in this Study as flood affected within the planning layer.
• Worked directly with affected residents to answer questions and provide support throughout the Study.
• Flood lot notations have been reviewed and some removed. For example, properties along the foreshore that do not experience overland flooding but are identified as flood-affected from rising water levels within large waterways (such as the Georges River or Port Hacking), have been removed because these properties have sufficient foreshore controls to protect development of these areas without additional lot flood notations, and this aligns with the Study’s methodology and intent.  
• Reviewed filtering parameters to ensure that very shallow or isolated ponding, and minimally impacted lots, are not included in the planning layer – in line with the intent and methodology outlined in the Study’s report.
• Confirmed the planning layer accurately captures continuous flow paths so that consistent controls apply along each flow path.
• Increased the Study area to capture flood risk for small urban areas within the four major catchments that were not included in the initial Draft Study.

Overland flow is where water from rainfall flows across the land on its way to major drainage systems, such as creeks and rivers. All properties will have some overland flow in heavy rain, even those located on hillsides. Overland flow becomes overland flooding when the flow is deep enough and/or running fast enough that it may pose a risk to life or property.

Overland flow flooding can occur when heavy rainfall is so intense that the stormwater drainage system cannot cope with the volume of water, or when drains become blocked. Stormwater systems are rarely able to be designed to cater for all rainfall events. When the amount of rainfall exceeds the capacity of the drainage system, excess water flows over the ground and follows the low points in the land as it makes its way toward creeks and rivers.

Ideally, this water travels along roads and gutters, however at low points when drains are at capacity water can build up, overtop the road verge and flow across parks, public land or private property. In many cases there are drainage easements that allow for overland flow, but water can also pass around buildings, fences and other obstacles. In some cases, water may enter homes, garages or basements.

Overland flow flooding, sometimes referred to as flash flooding, occurs most often during sudden and intense rainfall events with little warning. Although it is typically short-lived and usually subsides once rainfall eases, it can disrupt traffic, isolate properties and cause damage to buildings located within flood paths or not elevated sufficiently above expected flood levels.

My property hasn't been flood affected before. Why is my property identified in this Study?

Flood studies identify land that may be impacted in relatively rare events. Just because you have not personally experienced flooding at your home before does not mean that it is not at risk. Every flood is different and how the flood behaves will depend on many variables which are considered as far as possible when floods are modelled.

The Overland Flow – Flood Study models various rainfall scenarios, with two scenarios used to identify risk and help owners, developers, Council and Emergency Services plan for the future.

The first of these is known as the 1% Annual Exceedance Probability (1%AEP) flood. This is a flood that has a 1 in 100 chance of occurring at any time.

We use this to determine a Flood Planning Area (FPA) which uses the 1%AEP flood level and adds a 'freeboard' allowance, which provides an additional margin of safety to reduce the likelihood of a home or business becoming flooded.

We also use the Probable Maximum Flood (PMF) which represents the largest possible flood event that could occur in a particular area. While extremely rare, the PMF is used to understand the maximum extent of flooding and identify risks to life and critical infrastructure during extreme events.

The Probable Maximum Flood (PMF) represents the largest flood that could realistically occur in a particular area. It is an extremely rare event resulting from the most severe combination of rainfall and catchment conditions considered physically possible. While extremely rare, some floods in Australia have approached the scale of a PMF event.

The PMF is used to understand the maximum extent and depth of flooding that could be experienced and to identify potential risks to life, property and critical infrastructure during extreme flood events. The PMF extent is generally considered to define the full extent of the flooding and areas outside of this are not considered flood affected.

The PMF is used by councils, engineers and emergency management agencies to assess extreme flood risks, support emergency planning, consider the safety of evacuation routes and inform decisions relating to vulnerable or sensitive development.

The Flood Planning Area (FPA) uses the 1%AEP flood level and adds a 'freeboard' allowance, which provides an additional margin of safety to reduce the likelihood of a home or business becoming flooded.

Flood risk management is the process of understanding where flooding may occur, how severe it could be and what can be done to reduce its impacts on people, property and infrastructure.

The process starts with a Flood Study, which uses computer modelling to understand how floodwaters behave during different rainfall events. These studies help identify areas that may be affected by flooding now and into the future.

A Flood Risk Management Study then assesses the potential risks and investigates options to reduce flood impacts. This may include measures such as drainage improvements, flood mitigation works, planning controls or community education.

The findings are used to prepare a Flood Risk Management Plan, which helps Council prioritise actions based on their cost, feasibility and the benefits they provide to the community.

Completing this process in accordance with the NSW Flood Risk Management Framework may also help secure government funding for future flood mitigation projects.

The Flood Risk Management program process is also outlined below:

Once a Flood Risk Management Plan is completed, we review old Studies and Plans to update them with the latest information and review how effective they were and make any new recommendations on how to further reduce flood risk.

For more information on flood risk Studies and Plans we are working on, and to view completed studies, please visit our Flood Risk Management page.

The 1% Annual Exceedance Probability (1%AEP) flood is a flood that has a 1% chance of occurring in any given year. It was previously referred to as the "1 in 100 year flood", however this does not mean it only happens once every 100 years. A 1%AEP flood could occur more than once in a short period of time or not occur for many years. The term simply describes the chance of a flood of that size occurring in any year.

The 1%AEP flood is the most commonly used flood event by councils, engineers and planners and serves as the benchmark for assessing flood risk and planning future development.

Council’s stormwater system includes drains, pits, pipes and open channels. Like all underground drainage, it is designed to handle smaller, more frequent rain events so people aren’t dealing with puddles every time it rains. During larger storms, the system cannot capture all the water. When this happens, excess water flows over the ground, this is 'overland flow'. Roads and valleys are often the main paths that this water follows, and open channels can often manage larger flows, but these cannot be placed everywhere.

It is not always possible or cost-effective to upgrade the drainage system to handle every storm. However, Flood Studies help identify problem areas where improvements may be needed. Follow up Flood Risk Management Studies & Plans can then assess whether upgrades to drainage could reduce flooding and improve safety.

Ideally, excess water flows along roads, through parks or along drainage easements. However, water will always follow the lowest points in the landscape. These low areas are often where natural creeks or drainage paths existed before development.

Where these flows are not well managed, it is important to identify them so that options for improvement can be explored.

A 'Lot' is the area of land within the boundaries shown on a land title.

Some lots have a single home or business, while others may contain multiple separately owned units, apartments, villas, townhouses or businesses.

Flood studies generally assess flood risks based on the lot because buildings and land use can change over time. This provides a consistent way to understand and manage the flood risk now and into the future.

As a result of this, the number of Lots identified can differ from the number of homes, businesses and owners within the study area.

Being in either the Flood Planning Area (FPA) or Probable Maximum Flood (PMF) extent does not prevent development and importantly, only the part of a Lot identified as affected by flooding is considered when planning development.

If your property is identified in the Study, additional planning and design requirements may apply to ensure the development can safely withstand flooding and does not increase flood risk for neighbouring properties.

These requirements are outlined in Chapter 40: Environmental Risk – Flood Risk Management of the Sutherland Shire Development Control Plan (DCP). Depending on the type of development and the nature of the flood risk, considerations may include floor levels, building design, structural resilience, vehicle access and evacuation.

We recommend speaking with Council early in the planning process to understand any flood-related requirements that may apply to your property.

Flood behaviour can change over time as new studies, improved modelling techniques and additional survey information become available. As a result, areas identified as being affected by flooding may change as flood studies are updated.

Where flood information is available from adopted (not draft) studies, a notation may be included on planning certificates like other potential hazards such as bushfire, landslips, coastal hazards or contamination. These help owners, prospective purchasers and developers understand potential flood risks and any planning considerations that may apply to a Lot.

A property may be identified as flood affected if it is located within the Flood Planning Area (FPA)*, an area used to help guide planning and development. The FPA applies development controls to protect future development of all kinds from the known risks of flooding on a Lot.

The Flood Planning Area sits within the broader Probable Maximum Flood (PMF) extent.  This very rare event is used primarily for emergency planning purposes. However, controls are applied for sensitive developments including where vulnerable populations who cannot easily evacuate or respond to emergencies are located. For example, childcare, aged care, health services, or educational facilities. Hazardous facilities that may pose an environmental risk if flooded are also subject to controls.

Being in either the FPA or PMF extent does not prevent development and importantly, only the part of a Lot identified as affected by flooding are considered when planning development.

The study is not expected to have any impact on your rates. Rates help fund local infrastructure and services based on the property values set by the Valuer General.

Property values are influenced by a broad range of factors. There is no clear evidence that the adoption of overland flow information on its own would result in a sustained impact on property values.

Each insurance provider has its own risk assessment process used to set premium prices. The Insurance Council of Australia advises that insurers already hold extensive flood risk data. The industry has developed the National Flood Insurance Database (NFID), which insurers use to assess flood risk. In determining insurance premiums, insurers consider NFID data alongside other factors, including building type, location, and claims history.

We cannot rule out that Council's revised draft Overland Flow - Flood Study could impact premiums, however, it is likely it provides more accurate detail than studies conducted by an insurance company.

The standard insurers' definition of flood in Australia is defined in the Insurance Council of Australia's General Code of Practice as: "The covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal or dam."

Overland flow does not fit into this definition and is sometimes considered as ‘stormwater’ by insurers. However, it is recommended that you speak with your insurer for a full understanding of policy coverage and other types of flood risk at your property.

Further information on flood insurance is available from the Insurance Council of Australia:

Flood insurance explained - Insurance Council of Australia

Floodplain Management Australia partnered with the Insurance Council of Australia in developing three factsheets on flood insurance:

• Sharing Flood Risk Information with Insurers Fact Sheet - ICA
• Flood Insurance Pricing Fact Sheet - ICA
• Insurer Flood Data Requirements Fact Sheet - ICA

What is a Planning Certificate?

A Section 10.7 Planning Certificate provides landowners, prospective buyers and developers with information about the planning controls that apply to a Lot. This includes information about land use zoning, development controls and known constraints affecting the land, such as flood risk. Similar notations may also apply for other hazards, including bushfire, landslip, coastal hazards and land contamination.

A notation on a Planning Certificate does not prevent development from occurring. It simply indicates that development on the land may be subject to specific planning controls. If development is proposed, such as a new dwelling, extension or renovation, the flood risk may need to be considered to ensure future occupants can live safely and flood risks are appropriately managed.

Please note that no notations will be applied to planning certificates until the Study is adopted by Council.